Preserve mtime explicitly when creating tar artifacts (#18091)

* Preserve mtime explicitly when creating tar artifacts

When building tar archives, explicitly set mtime. This avoids losing that
information in the minitar `Writer.add_file_simple` method
 https://github.com/halostatue/minitar/blob/a531136b17b9efdddf0a0f39537845b454c2371e/lib/minitar/writer.rb#L139

* Better default for mtime

If for some reason we cant get a time from a File object (very unlikely)
a better default is Time.now rather than 0.

* Update rakelib/artifacts.rake

Co-authored-by: João Duarte <[email protected]>

---------

Co-authored-by: João Duarte <[email protected]>

Author: donoghuc

rakelib/artifacts.rake

@@ -669,7 +669,7 @@ namespace "artifact" do
       elsif stat.symlink?
         tar.symlink(path_in_tar, File.readlink(path), :mode => stat.mode)
       else
-        tar.add_file_simple(path_in_tar, :mode => stat.mode, :size => stat.size) do |io|
+        tar.add_file_simple(path_in_tar, :mode => stat.mode, :size => stat.size, :mtime => (stat.mtime || Time.now).to_i) do |io|
           File.open(path, 'rb') do |fd|
             chunk = nil
             size = 0